Following up an earlier report on narrow lines in early ER7 data ("ER7A"), attached are spectra and a longer list of lines found in the 5-2000 Hz band from examining an inverse-noise-weighted average spectrum over 149 hours of full-ER7 DC-readout data, using 30-minute Hann-windowed FScan SFTs. The overall spectrum is similar to before, but the additional data reveals more structure. Here are highlights and lowlight of apparent changes:
Thanks Keith!
Regarding the OMC dither lines, we had been running with the dithering enabled, but for nearly all of ER7 the error signals for the OMC-ASC loops were derived from the QPD signals. So, the dither lines were injected, but not suppressed. This choice was made because the dither signals acquire a large pitch offset when we run at high power, enough that using those error signals can saturate the OMC-SUS as the alignment loops try to correct. (The reason for the offset is not understood, maybe due to some junk light on the other QPDs used in HAM6 alignment that is not significant at low power.) Our preferred solution is to use something other than the OMC-SUS to center the beam, and switch to the dither loops for alignment. I wanted to keep the dither lines present to monitor the alignment drift during the run, this is something I haven't had a chance to follow up.
Before ER8 we will either switch to the dither signals for OMC alignment and move the frequencies above 2kHz, or decide to run on the QPD signals and turn off the dithering. Either way these nefarious lines won't be in the band. :)